1. Field of the Invention
The present invention relates to a spin MOSFET and a reconfigurable logic circuit.
2. Related Art
In recent years, MRAM (Magnetic Random Access Memory) with a MTJ (Magnetic Tunnel Junction) having a tunnel barrier layer interposed between two vertical magnetization films has been drawing attention, for its lower spin injection current density and excellent thermal stability. In this specification, a “vertical magnetization film” is a ferromagnetic film that has a magnetization direction (a magnetization easy-axis direction) substantially perpendicular to its upper face.
Meanwhile, intensive research and development are being made with respect to devices having novel functions such as spin MOSFETs. One of those devices is a spin MOSFET having source/drain regions made of a magnetic material. A spin MOSFET is characteristically capable of controlling its output properties simply by reversing the direction of the spin moment of the magnetic material of the source/drain regions. By taking advantage of this feature, it is possible to form a spin MOSFET having a reconfigurable function and an amplifying function, or a reconfigurable logic circuit (see APL84 (2004) 2307, for example).
As a spin torque transfer switching method, a write method involving spin injection has been suggested. A spin torque transfer switching can be seen by spin-injecting spin-polarized current. Also, to apply spin injection writing to spin MOSFETs, a structure having a MTJ in one of the magnetic bodies forming the source/drain regions has been suggested (see JP-A 2008-66596 (KOKAI)). With the use of the structure disclosed in JP-A 2008-66596 (KOKAI), the following effects can be achieved:
1) Two outputs can be used for reading; and
2) A spin torque transfer switching can be utilized, since a MTJ (a ferromagnetic stacked film) is provided.
Also, the use of vertical magnetization films in the MTJ in the source/drain region in a spin MOSFET has been suggested by the inventor, and has been filed for a patent by the applicant (Japanese Patent Application No. 2008-191146). In this spin MOSFET, source/drain regions are provided at a distance from each other in a semiconductor substrate, and a gate electrode is formed on a semiconductor region to be the channel region between the source region and the drain region. A ferromagnetic stacked film having a vertical magnetization film is provided on each of the source/drain regions, and at least one of the ferromagnetic stacked films on the source/drain regions has a MTJ structure (hereinafter also referred to as a lateral structure). With this structure, the spin-injection write current density can be lowered, the areas of the source/drain portions can be reduced, and thermal stability can be advantageously secured in a spin MOSFET.
In a case where vertical magnetization films are used in the MTJ of the source or drain region, however, a leakage magnetic field generated from the fixed magnetization layer of the MTJ affects the magnetic recording layer, and shift adjustment cannot be performed, as will be described below. Such a leakage magnetic field also affects adjacent spin MOSFETs. In such a spin MOSFET having a lateral structure including vertical magnetization films, spin relaxation in the semiconductor region to be the channel region is accelerated due to the leakage magnetic field. This problem was not recognized in the past, and was recognized first by the inventor.
As described above, in a spin MOSFET using vertical magnetization films in the ferromagnetic material of the MTJ in the source/drain region, the spin-injection write current density becomes lower, and excellent thermal stability and the likes can be achieved. However, a spin MOSFET having a lateral structure has the following problems: difficult field shift adjustment, influence of a leakage magnetic field on adjacent transistors, and accelerated spin relaxation in the semiconductor region to be the channel region due to the leakage magnetic field.